1. Technical Field of the Invention
The present invention relates to a drive circuit for a switching element, and in particular, to a drive circuit which controls a switching element targeted for being driven.
2. Related Art
This type of drive circuit is well known. For example, a patent document JP-3558324-B discloses a gate driving device. In the gate driving device, a Zener diode and a transistor are connected between the emitter and the gate of an insulated gate bipolar transistor (IGBT) to clamp the gate voltage to a predetermined level. The transistor is turned on when a detection value (voltage signal) of the collector current of the IGBT exceeds a threshold voltage. Thus, when it is judged that the collector current of the IGBT shows a value that may impair the reliability of the IGBT, the gate voltage can be clamped to a specified value, and further, the collector current can be limited.
Further, recently, power conversion circuits, such as on-vehicle inverters, deal with high current. In order to cope with high current, power conversion circuits in which a plurality of switching elements are arranged in parallel (parallel connected) have been developed and put into practical use. Using such a power conversion circuit, the current that can be dealt with is multiplied. Use of such a parallel connection may however involve detection of the collector current for each of the switching elements.
On the other hand, it is well known that an integrated circuit is used to configure a means for performing drive control of switching elements, or for clamping gate voltage. When such an integrated circuit is used for the parallel connected switching elements as mentioned above, the integrated circuit is required to have a plurality of terminals for inputting detection values of the respective collector currents.
In order to ensure universal use, an integrated circuit may be configured on the assumption of the use of the parallel connection mentioned above, and may be utilized even if the parallel connection is not actually used. When the integrated circuit is used for the case not using the parallel connection, some of the terminals for inputting detection values of the collector currents remain unused. The terminals not in use may be grounded (so as to have the same potential as that of the emitter of the IGBT), as disclosed in a patent document JP-H11-087909-A. Thus, the voltage of each of the terminals not in use is prevented from exceeding the threshold voltage and thus the function of clamping the gate voltage will not be exerted.
However, in the mode mentioned above, i.e., when the integrated circuit assumed to use parallel connection is utilized for the case not using the parallel connection, the occurrence of short circuit may raise a problem. Specifically, when short circuit occurs, for some reason, between the terminals for inputting detection values of the respective collector currents, the potentials of the terminals to which detection values of the collector currents are actually inputted are tend to be fixed to the ground potential. In this case, when the collector current becomes excessively large, the gate voltage cannot be clamped.